1. Field of the Invention
The present invention relates to multilayer films useful as packaging materials, particularly to packaging films that are oxygen permeable and thermoformable such as might be used in the packaging of products such as, for example, fresh poultry, frozen red meat, and certain produce.
2. Background of the Invention
Numerous film products are employed for packaging and for delivery of food products. These films were developed to have particular properties and often employ multiple layers to obtain the desired properties. For example, use of polyolefin based films characterized by high strength, excellent moisture and water vapor resistance, fair chemical resistance, and variable processability is well known.
Polyolefins often are used in combination with other polymers. No single polymer or copolymer can possess all the desired properties, however. Thus, films made from blends of polymers as well as multilayered structures have been developed. These have been found to provide a good balance of properties depending on the end use of the film. Many films designed for packaging applications in the food industry incorporate "barrier" polymers to prevent the passage of, for example, oxygen. The present invention, however, is directed to films to be used for packaging certain food products such as fresh poultry which must necessarily possess high oxygen permeability.
For many such packaging applications, thermoforming an oxygen permeable film is desirable. Most typically, a non-thermoforming film or web is used in combination with a thermoforming film or web to produce a final package. In a typical operation, a forming film is formed into a mold to provide a cavity in which a food product is placed. A non-forming film can be placed over the cavity and vacuum sealed, by means known in the art, to the periphery of the forming web. Many meat products are packaged in this manner.
Heretofore, only one commercially available thermoformable film having good oxygen permeability properties has been available. That is, thermoformable monolayer ionomer films are known to provide high oxygen permeability. Generally speaking, ionomers are metal neutralized salts of ethylene/acrylic acid or ethylene/methacrylic acid copolymers, commercially available from DuPont de Nemours (Wilmington, Del.). Although monolayer ionomeric films produced by a blown or cast process provide oxygen permeability and thermoformability, their failure rate is high because the periphery of the film must be heated to its softening point for sealing to occur. Thus, burn through (i.e., rupture of the film due to heat) is common.
Thus, a need in the art exists for a heat-sealable, thermoformable, oxygen permeable film or web which does not degrade upon sealing.